Medical grade cast CoCrMo alloy (F75) was ion nitrided under different process parameters including time (2, 9 hr) and temperature (600, 800 0C) at a gas mixture of 80%N2–20%H2. The nitrided surfaces were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). MicroVickers hardness tests were done on the cross sections of the nitrided specimens to investigate the hardness profile. The significant effects of nitriding temperature and time on the microstructure and hardness value of nitrided surface layers were displayed.